Increased use of biofuels can enhance energy and economic security, decrease life-cycle emissions of greenhouse gases and other environmental pollutants, and provide many social benefits. Biofuels impact transportation, defense, and aviation applications. Thermochemical and biochemical conversion pathways are two major routes for transforming solid biomass to liquid bio-oils, biofuels, or high-value chemical intermediates/products. Renewable liquid fuels are projected to experience the largest increase in production for meeting the United States energy consumption demands, an expected growth from 8% in 2010 to more than 14% of liquid fuels in 2035. The need for efficient separation processes is essential for moving toward a cost-competitive biofuel production process.
Significant challenges exist to integrate separation technologies into bio-oil conversion and downstream bio-refinery processes, including pretreating bio-oil. Similar cost reduction benefits may be realized for the thermochemical conversion of biomass to drop-in hydrocarbon fuels if advanced separation technologies are integrated into the intermediate processing steps within the various conversion pathways. These intermediate processing steps include fast pyrolysis, in situ or ex situ catalytic fast pyrolysis, hydropyrolysis, and hydrothermal liquefaction. Possible separation needs in the bio-oil thermochemical conversion pathway include (1) hot gas filtration and vapor-phase processing to minimize vapor cracking and light gas production, (2) solid/liquid filtration for char mitigation from liquid bio-oils, (3) fractionation of bio-oil condensates for specific catalytic transformation reactions, (4) selective separation of oxygenated/deoxygenated hydrocarbons from water or oxygenated hydrocarbons from deoxygenated hydrocarbons (either vapor- or liquid-phase) in intermediate upgraded bio-oils, and (5) selective removal of destabilizing components (e.g., organic acids) from bio-oil vapors or liquids. More efficient separation technologies in these areas would improve the overall efficiency of the biomass conversion process.
In most separation applications, the membrane performance objectives are high permeation flux, high separation selectivity, high stability (thermal, chemical, mechanical), high resistance to fouling for long-term operation, and scalability for membrane operation and material fabrication. A major challenge for integrating membranes into bio-oil or biofuel processing pathways is ensuring that the membranes maintain both high flux and high selectivity during long-term separations. Traditional membranes suffer from a permeability-selectivity “trade-off”, i.e., the increase in the selectivity comes at the cost of reduced flux, and vice-versa. This issue is apparent in zeolite-based membranes, which have a high selectivity, but are limited in permeation flux due to their small sub-nanometer pores.
Moreover, conventional membranes are generally incapable of withstanding the relatively high temperature range of 300-600° C. and high water vapor concentration of pyrolysis bio-oil processing operations. Polymer membranes will generally degrade at such high temperatures. Although some zeolite-based membranes can be more robust at such temperatures, they are known to suffer from low permeation flux and also from hydrothermal instability at elevated temperatures, particularly when a high water vapor concentration (e.g., ˜20-25 wt %) is present in the pyrolysis bio-oil vapor mixture. Thus, although integration of a robust vapor-separating membrane having both high flux and high selectivity into a pyrolytic system would provide significant advantages, no viable way for achieving this is currently known.